46908-AC3
New Catalysts for Phosphazene Ring-Opening Polymerization
The progress of ring opening polymerization (ROP) of cyclo-hexachlorophosphazene, catalysed by triethylsilyl carboranes at room temperature in o-dichlorobenzene solution, has been studied in detail by 31P NMR spectroscopy. This gives new insight into the mechanism of the ROP reaction. For the first time, end groups on the growing polymer have been observed. These end groups appear to be an N-bound cyclo-phosphazene at one end, and a P-bound cyclo-phosphazene at the other. During polymerization, larger ring cyclo-phosphazenes are observed. These include tetramer, pentamer, hexamer and heptamer. Higher cyclo-oligomers may be formed but are not distinguiishable from linear polymer by 31P NMR. A mass spectral method for the quantitative determination of low molecular weight fragments is being developed. Based on the essentially constant abundance ratios in 31P NMR as a function of time, it appears that the higher cyclo-oligomers are under thermodynamic control. A rapid "back biting" mechanism can be proposed to accommodate this observation. In the search for better catalysts than the trialkylsilyl carboranes, trialkylgermyl and trialkyltin carboranes have been synthesized. They will tested as ROP catalysts during the next grant period.
